Conventionally, in order to prevent electrostatic trouble and electrostatic adsorption due to electrostatical-charge of components, neutralizers are placed near working benches, conveyors and the like in a semiconductor device manufacturing line, a cell manufacturing process for mobile phones and the like. The neutralizers used in these working areas include neutralizers which: emit (irradiate) positive ions or negative ions onto a neutralization target object (a component) in which charges are unevenly distributed because positive charges or negative charges are excessive wholly or partially; and thereby electrically neutralize the object. These neutralizers are classified into some types depending on neutralization methods. Descriptions will be hereinbelow provided for characteristics of the methods.
(1) AC-type
An AC-type neutralizer is configured to apply a sine-wave high voltage (with a frequency of 50/60 Hz) to a single discharge needle and thus to cause the needle to alternately generate positive and negative ions. Because both positive and negative ions are generated from the single discharge needle, this type of neutralizer is characterized by having less temporal and spatial deviations of ion balance.
In this respect, “ion balance” indicates how much a residual potential on an object deviates from zero volts after ion irradiation. An idealistic characteristic is that the residual potential is stationarily equal to zero volts. In addition, the temporal deviation of ion balance means that, to while a neutralizer is continuously operated, the residual potential deviates due to differences between positive and negative discharge needles in the degrees of dirt adhesion, erosion and abrasion. On the other hand, the spatial deviation of ion balance means that, when neutralization target objects are irradiated with ions, the residual potentials differ depending on the positions of the neutralization target objects. The spatial deviation of ion balance is determined, as will be described later, by irradiating ions onto neutralization target objects that are located at predetermined distances from a neutralizer, and then by performing a measurement to find a place where a neutralization target object has a residual potential. Moreover, ion balance variation, to be described later, means that the potential on the surface of a neutralization target object periodically varies between positive and negative each time the object is irradiated with positive and negative ions alternately.
(2) DC-Type
A DC-type neutralizer is configured to apply a positive high voltage to a positive discharge needle and a negative high voltage to a negative discharge needle; and thus to cause each of the discharge needles to stationarily produce positive or negative ions. Positive and negative ions thus emitted are less likely to recombine with each other before reaching a neutralization target object. For this reason, the DC-type neutralizer is characterized by being capable of causing ions to travel farther than the AC-type neutralizer does.
(3) AC High-Frequency Type
An AC high-frequency type neutralizer is configured to apply a high-frequency voltage with a frequency of 20 kHz to 70 kHz to a single discharge needle. The AC high-frequency type neutralizer is characterized in that a transformer can be made lighter and smaller than that for the general AC-type neutralizer.
(4) Pulsed DC Type
A pulsed DC-type neutralizer is configured to alternately apply a positive high voltage to a positive discharge needle and a negative high voltage to a negative discharge needle; and thus to cause the discharge needles to alternately produce positive and negative ions. This type of neutralizer is characterized in that the temporal deviation of ion balance is improved as compared with the general DC-type neutralizer. Note that prior art related to the above is disclosed in Japanese Patent Application Laid-Open brochure, No. JP-A 2002-43092 (Patent Document 1).
(5) Pulsed AC Type
A pulsed AC-type neutralizer is configured to apply a rectangular-wave high voltage to a single discharge needle. This type of neutralizer is characterized by being capable of producing more ions than the general AC-type neutralizer does, and of varying its oscillatory frequency (see Patent Document 2). Note that prior art related to the above is disclosed in Japanese Patent Application Laid-Open brochure, No. JP-A 2000-58290 (Patent Document 2).
The foregoing types of neutralizers, however, have problems as follows.
(1) AC Type
A heavier and larger transformer needs to be used to generate a high voltage. As this type of neutralizer is often used while being placed on a working bench or being hanged, a compact and light neutralizer is desirable. However, it is difficult to build a smaller and lighter AC-type neutralizer. In addition, since positive and negative ions are alternately produced, a neutralization target object is charged positively and negatively in an alternate manner. This means the ion balance varies with time. As a result, the AC type neutralizer has difficulty in keeping the residual potential close to zero volts after ion irradiation. Moreover, the AC type neutralizer produces less positive and negative ions than the DC type neutralizer does, and thus is inferior to the DC type neutralizer in terms of the attenuation time characteristic and the neutralization range. Here, the attenuation time characteristic means a time until the potential of a neutralization target object falls into a tolerable level after ion irradiation. If a neutralizer can reduce the potential of a charged neutralization target object to the tolerable level at a shorter length of time, the neutralizer is better in the attenuation time characteristic. In addition, the neutralization range means a spatial range in which a neutralizer can reduce the potential of the neutralization target object to the tolerable level with ion irradiation.
(2) DC Type
During continuous operation, differences occur between the positive and negative discharge needles in the degrees of dirt adhesion, erosion and abrasion. This causes a temporal deviation of ion balance. In addition, depending on where the discharge needles are located, some places are more susceptible to positive ions, and others are more susceptible to negative ions. As a result, a neutralization target object located on each of such places is positively or negatively charged, and thus a spatial deviation of ion balance occurs.
(3) AC High-Frequency Type
This type of neutralizer produces positive and negative ions at short intervals, and thus the emitted positive and negative ions are likely to be recombined with each other before reaching a neutralization target object. This makes it difficult to cause ions to travel far. In addition, less ions reaching the object lead to deterioration in the attenuation time characteristic.
(4) Pulsed DC Type
As is the case with the DC type neutralizer, during continuous operation, differences occur between the positive and negative discharge needles in the degrees of dirt adhesion, erosion and abrasion, and thus a temporal deviation of ion balance occurs. In addition, a spatial deviation of ion balance occurs between a place susceptible to the positive discharge needle that is more likely to be fouled with dirt and a place susceptible to the negative discharge needle that is less likely to be fouled with dirt. As a result, this type of neutralizer positively or negatively charges the neutralization target object. Moreover, as alternately producing positive and negative ions, this type of neutralizer positively and negatively charges the neutralization target object in an alternate manner, like the AC type neutralizer. As a result, the ion balance varies from a temporal point of view.
(5) Pulsed AC Type
As alternately producing positive and negative ions, this type of neutralizer positively and negatively charges the neutralization target object in an alternate manner, and produces more ions than the AC type neutralizer does. For this reason, the ion balance varies from a temporal point of view.
As described above, the conventional types of neutralizers have problems in any of size, weight, attenuation time characteristic, or ion balance characteristic. Currently, there has been developed no neutralizer that overcomes all these problems.
The present invention has been made for solving the foregoing problems. An object of the present invention is to provide a compact and light neutralizer which is better in the attenuation time characteristic and the ion balance characteristic.